Metal amidoboranes are materials with applications in both chemistry and materials science. Their chemical applications, and those of the lithium derivative in particular, are mainly associated with their ability to reduce organic carbonyl compounds in solution. In the area of materials science, alkali metal amidoboranes, such as sodium or lithium amidoboranes, are considered as efficient hydrogen sources for low-temperature proton exchange membrane (PEM) fuel cells.
Fuel cell technology offers a unique chance to generate electricity in an essentially pollutant-free way by utilizing hydrogen generated from renewable sources. However, it can be only realized if there is an efficient way to safely store significant amounts of hydrogen at temperatures ranging from near ambient to about 90° C. and at pressures below 100 bar. In addition, an ideal hydrogen storage media should be able to release hydrogen at temperatures not exceeding 100° C. These requirements favor storing hydrogen in the form of hydrogen rich solids, rather than compressed or liquid hydrogen, which require high pressures (>700 bar) or low temperatures (20K), respectively.
Lithium amidoborane possesses a hydrogen content of 13.7% (by weight) and is an excellent hydrogen source for PEM fuel cells since the waste heat generated by the fuel cell can be utilized to free almost all of the material's hydrogen content in one step at ˜90° C. Although the hydrogen content of sodium amidoborane is somewhat lower (9.5%), it is still much higher than that of such conventional hydrogen sources as magnesium hydride (7.7%) or sodium aluminum hydride (7.5%). Thus, there is significant general interest in developing robust, commercially viable procedures for the preparation of these hydrogen sources.
A critical requirement for hydrogen gas derived from hydrogen sources used in fuel cell applications is purity; even low level contamination in the hydrogen gas generated from boron-based materials (e.g., ammonia borane, metal borohydrides, and metal amidoboranes) such as organic solvents, diborane, or borazine are capable of poisoning fuel cell catalysts. Furthermore, the presence of polar solvents such as tetrahydrofuran (THF), which may strongly coordinate to metal ions, can negatively influence chemical reactivity of metal amidoboranes. Therefore, it is important that any commercial process used to prepare borane-based materials for hydrogen storage applications yield high purity substances. Ideally, the hydrogen storage material would contain limited or no associated organic solvent molecules.
Metal amidoboranes have been prepared using a variety of methods, many of which employ ammonia borane as a starting material. For example, solution-based processes have been reported that comprise metallation of ammonia borane in THF with a strong alkali base such as butyllithium, lithium diisopropylamide, lithium hydride, sodium hydride, lithium amide, or sodium amide to give a corresponding sodium or lithium amidoborane in THF solution. These solutions can be used directly as reducing agents in chemistry applications, without attempts to isolate the sodium or lithium amidoborane. Although solid sodium or lithium amidoborane can be isolated by evaporating the solvent, THF and other ethereal solvents can be exceedingly difficult to remove from the metal amidoborane. Consequently, contemporary solution-based methods fail to produce solid metal amidoboranes of adequate purity at commercially relevant scales.
Another approach used for the preparation of metal amidoboranes involves ball-milling a mixture of solid ammonia borane and a metal hydride in the absence of a solvent to give the metal amidoborane. Since no organic solvents are used, the resultant metal amidoborane is free of organic solvent contaminants. However, significant amounts of flammable hydrogen gas are produced in the process (e.g., one molecule of H2 per each molecule of the metal hydride used), which leads to significant pressurization of the ball-milling vessel creating a potential explosion hazard. Moreover, the metal hydride-based materials prepared using the balling-milling approach tend to be difficult to handle because they are highly sensitive to air and moisture.
Thus, there is a need for efficient, safe, and cost-effective processes for the preparation of metal amidoboranes compositions of high purity and high hydrogen content.